1. Field of the Invention
The present invention relates to a method for performing the reassignment (channel reassignment) of TSA (Time Slot Assignment) at the high-rate signal side, uninterruptedly, in a SONET/SDH optical transmission system, and a circuit for implementing such a method.
2. Description of the Related Art
In recent SONET/SDH optical transmission systems, the need for higher-speed, higher-capacity transmission apparatuses is increasing with increasing demand for communication channels in the information society. On the other hand, the recent rapid development of information services such as the Internet and electronic mail has been supported by increased speeds and increased capacities of IP routers and other equipment indispensable for the processing of such services. With such high-capacity equipment located at the primary-rate signal side, optical transmission apparatuses that perform multiplexing must handle correspondingly larger-capacity signals. Accordingly, the capability to flexibly cope with the increased capacity of equipment at the primary-rate signal side is particularly required of recent optical transmission apparatuses and systems.
In this case, since the concatenation of channels to accommodate large-capacity primary-rate signals cannot be performed on a plurality of arbitrary channels, there are certain cases in which the channel assignments at the higher rate signal side must be changed.
For example, consider the case where an OC-3 (or STS-3c) is to be accommodated on an OC-12 having 12 channels each accommodating a SONET OC-1 (or STS-1). When accommodating an OC-3 (STS-3c) on an OC-12, a concatenated area for accommodating the OC-3 must be created by concatenating CH1-CH3, CH4-CH6, CH-7-CH9, or CH10-CH12. Therefore, if a new STS-3c is to be accommodated in a section where the positions shown by hatching in FIG. 1 are occupied as shown, the signal occupying CH8 must be reassigned and moved to CH6 in order to create a concatenated area for accommodating the new STS-3c, as shown in FIG. 2. By changing the assignment in the section where the new STS-3c is to be accommodated, as shown in FIG. 2, CH7-CH9 can be concatenated to accommodate the STS-3c.
In prior art optical transmission systems, by performing such TSA reassignment (channel reassignment) in accordance with the following sequence of operations, the channel reassignment can be accomplished without having to cut the channels currently used for services for a long period of time.
(a) In the transmitting optical transmission apparatus, the signal on the source channel (CH8 in the above example) is also output onto the destination channel (CH6 in the above example). [Bridge setting]
(b) In the receiving optical transmission apparatus, the signal receiving channel is changed from CH8 to CH6. [Switching]
(c) In the transmitting optical transmission apparatus, transmission of the signal onto CH8 is stopped. [Bridge clearing]
In SONET/SDH optical transmission systems, pointer circuits are provided to prevent data dropout or duplication by absorbing minute differences in clock frequencies between different optical transmission apparatuses. The pointer circuit has an ES (elastic store) memory for temporarily storing payload data, and absorbs the minute differences of clock frequencies by changing the payload insertion position in a frame at the transmitting end based on the phase difference between its write address and read address.
The pointer processing by the pointer circuit is performed independently for each channel, except between concatenated channels. As a result, even if the phase of the payload (J1 phase) at the transmitting end is kept aligned between the source channel and the destination channel after the bridge setting in (a), the J1 phase at the receiving end will be shifted after undergoing the pointer processing at each intermediate node.
Accordingly, because of the switching operation in (b) at the receiving end, duplicated transmission of data or data dropout will occur, resulting in a momentary interruption of the signal. This momentary signal interruption is caused, not due to the switching operation itself, but due to the pointer processing along the path to the receiving end, and therefore cannot be avoided even if the switching operation usually controlled with asynchronous timing is synchronized to the main signal timing. Further, since the data duplication and dropout described above are inherently due to a difference in pointer value, restoration from the interruption must wait until the normal pointer of the channel after the switching is received three times in succession.